Panels of gypsum wallboard having a core of set gypsum have long been used as structural elements in the fabrication of buildings. Such panels, also commonly known as “wallboard,” “drywall,” or “plasterboard,” are typically used to form the partitions or walls of rooms, elevator shafts, stairwells, ceilings and the like and represent a less costly and more expeditious alternative to conventional plaster walls.
In its most common adaptation, gypsum wallboard is produced by sandwiching a solid gypsum core made from an aqueous slurry of calcined gypsum, usually a slurry of calcium sulfate hemihydrate, between two sheets of a facing material, typically heavy papers. Gypsum wallboard is manufactured continuously at a high speed by continuously depositing the aqueous slurry of calcined gypsum and other ingredients onto one of the two facing sheets and then bringing the second facing sheet into contact with the free surface of the gypsum slurry to form a sandwich-like structure.
Various types of facing materials are known in the art. Multi-ply paper is most commonly used. As an alternative to paper facing sheets, gypsum wallboard can also be manufactured with a fibrous mat (such as a glass fiber mat) and with coated fiber mats as the facing material. Examples of such wallboards include those described, for example, in U.S. Pat. Nos. 4,647,496 and 7,807,592. In addition to improved water resistance, fibrous mat facing materials often provide other significant improvements in strength, performance and other physical attributes.
The calcined gypsum slurry deposited between the two facing sheets, then sets (i.e., the calcined gypsum reacts with water from the aqueous slurry) to form a rigid board-like structure. The so-formed board then is cut into panels of a desired length (for example, eight to sixteen feet). Because the so-formed board contains excess water (water is necessary not only for hydrating the calcined gypsum but also to ensure sufficient fluidity of the gypsum slurry during preparation of the board), the board must then pass through a drying kiln in which excess water is removed and the gypsum wallboard is brought to a final hydrated, but dry state. After the core has been set and is fully dried, the sandwich becomes a strong, rigid, fire-resistant building material.
Certain lightweight gypsum wallboards have recently been described in which the gypsum core is formed from a foamed gypsum slurry comprising calcined gypsum, a pre-gelled starch, naphthalenesulfonate and preferably a trimetaphosphate salt, see U.S. Pat. No. 7,731,794.
Nonetheless, an alternate approach for making a high strength, lightweight wallboard product, which has the structural integrity to withstand the structural and industrial requirements of traditional, heavyweight wallboard products, yet avoids the use of costly naphthalenesulfonate would be useful.
Such a strong, lightweight wallboard should satisfy industry criteria, such as ASTM C-1396, should be cost-effective to manufacture, and should have a strength at least equal to previously known and disclosed wallboard products while reducing the weight of the wallboard significantly. The present invention is directed toward meeting such criteria.